Magnetic inductive flow meters, whose mode of operation is based on the principle of electromagnetic induction (=Faraday's law of induction), have been known for many years and are extensively being used in industrial measurement engineering. According to the law of induction, an electric field strength that is perpendicular to the direction of flow and perpendicular to the magnetic field is generated in a flowing medium, which is carrying charge carriers, that flows through a magnetic field. In magnetic inductive flow meters, the law of induction is exploited in such a way that a magnetic field is generated by means of a magnetic field generating device, which commonly has two energized magnetic coils, the magnetic field being passed at least in part through the measuring tube, wherein the magnetic field that is generated has at least one component that is perpendicular to the direction of flow. Within the magnetic field, every volume element of the flowing medium, which passes through the magnetic field and has a specific number of charge carriers with the field strength that arises in this volume element, contributes to a measuring voltage that can be tapped by the electrodes.
Since the induced voltage tapped by means of the electrodes is proportional to the rate of flow of the medium averaged over the cross section of the measuring tube, the volume flow can be directly determined from the measured voltage if the diameter of the measuring tube is known. The only prerequisite for the use of a magnetic inductive flow meter is a minimum conductivity of the medium. In addition, it must be ensured that the measuring tube is filled with medium so that the level of the medium is at least higher than that of the measuring electrodes. As, however, a substantial error can occur depending on the fill level, if the measuring tubes are not completely filled, magnetic inductive flow meters are primarily optimally suited for uses in which the measuring tube is completely filled. For this reason, in practice, in most cases magnetic inductive flow meters have a measuring device for empty pipe detection, which indicate to the user when the fill level has dropped so much that the measured value can no longer be determined with the required degree of accuracy. This may already be the case, for example, in the case of a measuring tube that is only two-thirds filled so that the measuring devices used in practice for “empty pipe detection” will not only generate a signal when the measuring tube is actually “empty”.
If a magnetic inductive flow meter should also display a measured value when it is not completely filled, the degree to which it is filled must also be known so that the measured value can be corrected. Such correction values and the determination thereof are discussed in DE 196 37 716 C1 for example. To this end, a test signal voltage is applied to a pair of electrodes that are located opposite one another and a reaction voltage generated thereby on a second pair of electrodes that are located opposite one another is measured, wherein the two pairs of electrodes are coupled with the medium. A correction value is determined on the basis of the determined relationship between the reaction voltage and the test signal voltage, wherein corresponding correction values are first empirically determined and stored in a memory.
The degree to which the measuring tube is filled can also be capacitively determined according to DE 196 55 107 C2. In so doing, the two electrodes for measuring the flow of the medium are capacitively coupled with the medium, wherein the electrodes form a capacitor with the medium as a dielectric medium. In so doing, on the one hand, the voltage induced in the medium is tapped as a measure for the speed of flow of the medium by means of the two electrodes, and, on the other hand, the electrodes are supplied with an alternating voltage, and the capacitance between the electrodes is determined by means of a control and evaluation circuit as a measure of the percentage of the conductive medium in the measuring tube.